This invention relates to the transmission of sonic signals, and more specifically, to transducers for transmitting such signals through the air.
Ultrasonic signals are sound waves of frequencies above the audible range (generally 20 kHz). Many, if not most applications involving ultrasound require generation of a well-defined beam. Accordingly, ultrasonic transducersxe2x80x94which convert electrical signals into corresponding acoustic signalsxe2x80x94should have highly directional transmission characteristics in addition to high conversion efficiency. Furthermore, the mechanical impedance of the transducer should match, as closely as practicable, the impedance of the propagation medium.
Two important classes of ultrasound transducer for transmission through air are electrostatic and piezoelectric crystal devices. In an electrostatic transducer, a thin membrane is vibrated by the capacitive effects of an electric field, while in a piezoelectric transducer, an applied potential causes the piezo ceramic material to change shape and thereby generate sonic signals. Both types of transducer exhibit various performance limitations, which can substantially limit their usefulness in certain applications. In particular, these performance limitations have inhibited the development of parametric loud-speakers, i.e., devices that produce highly directional audible sound through the nonlinear interaction of ultrasonic waves. In a parametric system, a high-intensity ultrasonic signal that has been modulated with an audio signal will be demodulated as it passes through the atmospherexe2x80x94a nonlinear propagation mediumxe2x80x94thereby creating a highly directional audible sound.
Piezoelectric transducers generally operate at high efficiency over a limited bandwidth. In parametric applications the degree of distortion present in the audible signal is directly correlated with the available bandwidth of the transducer, and as a result, the use of a narrowband (e.g., piezoelectric) transducer will result in sound of poor quality. Piezoelectric transducers also tend to have high acoustic impedances, resulting in inefficient radiation into the atmosphere, which has a low impedance. Because of this mismatch, most of the energy applied to the transducer is reflected back into the amplifier (or into the transducer itself), creating heat and wasting energy. Finally, conventional piezoelectric transducers tend to be fragile, expensive, and difficult to electrically connect.
A conventional electrostatic transducer utilizes a metallized polymer membrane held against a conductive backplate by a DC bias. The backplate contains depressions that create an acousto-mechanical resonance at a desired frequency of operation. An AC voltage added to the DC bias source alternately augments and subtracts from the bias, thereby adding to or subtracting from the force drawing the membrane against the backplate. While this variation has no effect where the surfaces are in contact, it causes the membrane to vibrate above the depressions. Without substantial damping the resonance peak of an electrostatic transducer is fairly sharp, resulting in efficient operation at the expense of limited bandwidth. Damping (e.g., by roughening the surface of the membrane in contact with air) will somewhat expand the bandwidth, but efficiency will suffer.
Another technique for expanding the bandwidth of an electrostatic transducer, as described in Mattila et al., Sensors and Actuators A, 45, 203-208 (1994), is to vary the depths of the depressions across the surface of the transducer so as to produce different resonances that sum to produce a wide bandwidth.
The maximum driving power (and the maximum DC bias) of the transducer is limited by the size of the electric field that the membrane can withstand as well as the voltage the air gap can withstand. The strongest field occurs where the membrane actually touches the backplate (i.e., outside the depressions). Because the membrane is typically a very thin polymer film, even a material with substantial dielectric strength cannot experience very high voltages without charging or punchthrough failure. Similarly, because the use of a thin film means that the metallized surface of the film will be very close to the backplate, the electric field across the film and hence the capacitance of the device is quite high, resulting in large drive-current requirements.
Piezoelectric film transducers utilize light, flexible membrane materials such as polyvinylidene fluoride (PVDF) film, which changes shape in response to an applied potential. The film can be made very light to enhance its acoustic-impedance match to the air, resulting in efficient ultrasonic transmission. In one known configuration, a PVDF film is coated on both sides with a conductive material and placed atop a perforated metal plate. The plate represents the top of an otherwise closed volume, and a vacuum applied to the volume draws the membrane into the perforations. An AC voltage source connected across the two metallized surfaces of the membrane (which act as electrodes separated by a dielectric) causes the PVDF material to expand and contract, varying the degree of dimpling into the perforations and thereby causing the generation of sound waves. In a related configuration, also known, the membrane is disposed beneath the perforated plate rather than above it, and a pressure source is substituted for the vacuum. In this version, the AC source varies the degree to which the membrane protrudes into or through the perforations, once again creating sound.
While the electro-acoustic characteristics of these transducers render them suitable for parametric applications, their practicality is questionable. It is unlikely that the vacuum or pressure can be adequately maintained for long periods in commercially realistic environments, and any slight leakage will cause the transducer to lose sensitivity and eventually fail.
In accordance with a first aspect of the present invention, the maximum power output of an ultrasonic transducer is not limited by the dielectric strength of the transducer membrane. Rather than placing the membrane directly against the surface of a conductor as in conventional devices (whereby the electric field across the membrane is very large), it is instead held against a dielectric spacer. The transmission of ultrasound does not depend on the presence of a powerful electric field. Accordingly, relatively large bias and driving voltages can be applied across the membrane and spacer without risk of failure, because the spacer substantially reduces the electric field experienced by the membrane. Moreover, because the spacer also reduces the capacitance of the transducer, the driving current requirements are correspondingly reduced, simplifying design of the power amplifier.
A sonic transducer in accordance with this aspect of the invention may include a conductive membrane, a backplate comprising at least one electrode and, disposed between the membrane and the backplate, a dielectric spacer comprising a series of depressions arranged in a pattern, the depressions forming cavities each resonant at a predetermined frequency. The depressions may take any suitable form, e.g., annular grooves arranged concentrically, a pattern of distributed cylindrical depressions, etc., and may extend partially or completely through the dielectric spacer. Moreover, the depressions may vary in depth through the spacer in order to form cavities resonant at different frequencies; a different electrode may be assigned to each set of depressions of a single depth.
In a second aspect, the invention combines both piezoelectric and electrostatic modes of operation. A sonic transducer in accordance with this aspect of the invention may comprise a substantially nonconductive piezoelectric membrane having a pair of opposed conductive surfaces, a backplate comprising at least one electrode, and means for creating a resonant cavity or structure between the membrane and the electrode(s). For example, the cavities may be formed by a dielectric spacer having depressions (such as cylindrical recesses or apertures, grooves, etc.) and disposed between the membrane and the electrode(s). A DC bias urges the membrane into the resonant cavities and an AC source, connected across the membrane, provides the driving signals.
The transducers are preferably driven with circuits in which the capacitive transducers resonate with circuit inductances at the acoustical-mechanical resonant frequencies of the transducers. This provides a very efficient transfer of electrical energy to the transducers, thereby facilitating the use of relatively high carrier frequencies. The efficiency and versatility of the transducers described herein makes them suitable for parametric as well as other ultrasonic applications such as ranging, flow detection, and nondestructive testing. In parametric applications, a plurality of transducers may be incorporated into a transducer module and the modules are arranged and/or electrically driven so as to provide, in effect, a large radiating surface and a large nonlinear interaction region.